The present invention refers to a device for an electric fence, including an electric unit, which is connectable to an electrically conducting fence and includes a voltage generator arranged to apply a relatively high electric voltage to said fence, a proximity detector, which is arranged to sense a state at which an animal is in contact with or in the proximity of said fence, and a control unit, which is connected to the voltage generator and the proximity detector and arranged to activate the voltage generator to apply said relatively high electric voltage to said fence in response to said state.
The invention is primarily directed to electric fences for protecting plant arrangements and plantations, such as kitchen gardens, flower bed, single plants, and smaller gardens, from being entered and destroyed by animals, for instance roe deer, hares, rabbits, etc. Although the device to be described primarily is adapted to smaller plant arrangement, it is to be noted that the solution according to the invention also is applicable to electric fences for defining the space of movement for cattle, such as for instance sheep, cows, horses, etc. Also other areas of application than electric fences are possible for the device according to the invention.
Electric fences are well known for preventing animals from entering or leaving in an area. Conventional electric fences operate in such a manner that they at uniform time intervals, for instance once per second, applies a high voltage pulse to the electric fence. Normally a transformer is utilised, the primary winding of which is connected to a batter of any other voltage source via a switch. The switch is closed during for instance on second and is thereafter opened, wherein a high voltage pulse is formed at the secondary winding of the transformer which is connected to the electric fence and to ground via a so-called earth spit.
The disadvantage of such conventional electric fences are the high consumption of effect since the high voltage pulses are generated continuously irrespective of if an animal touches the fence or not. This means that a batter which is utilised for powering the electric fence frequently has to be charged, which is a time and labour consuming operation. In order to overcome this problem it has been proposed to use proximity detectors in order to enable application of a high voltage only when an animal is in contact wit the electric fence. Such proximity detectors are known from the patent literature. For instance, FR 2 604 324EP 177 899 and U.S. Pat No. 3.655,995 disclose electric fences with proximity detectors. These known proximity detectors are based on the principal that a relatively low direct voltage is applied to the electric fence and that the resistance between the electric fence and ground is sensed. When the electric fence is free from contact to the ground, the resistance is thus very high and when an animal touches the electric fence the resistance sensed decreases, which may be detected by means of the proximity detector. When a reduced resistance has been detected a high voltage, which is applied to the electric fence, is generated.
However, such proximity detectors have a relatively low sensitivity since the resistance still may be relatively high when an animal for instance only touches easily the electric fence and/or when the ground and the animal in question are relatively dry. Furthermore, humid grass or other humid plants, which touch the electric fence, may reduce the resistance so much that high voltage will be generated. A further problem with this type of proximity detectors is that oxide formation on the electric fence leads to an increase of the resistance.
FR p2 725 870 discloses another type of electric fence having a condenser which is charged with a high voltage and when an animal touches the electric fence the condenser is discharged.
Consequently, this document discloses no proximity detector in a proper sense.
FR 2 673 020 discloses an electric fence having an electric conduit which forms a closed loop between two terminals of a unit which is arranged to generate high voltage pulses. The device disclosed includes a detecting unit which is arranged to detect the presence of an intruder by sensing a variation in the capacitance.
The object of the present invention is to provide an electric fence having a high detecting reliability and a low energy consumption.
This object is obtained by the device initially defined, which is characterized in that the proximity detector is arranged to detect said state by sensing an electric parameter, which substantially depends on the capacitance between said fence and ground. In such a manner it is possible to apply high voltage pulses only when the presence of an animal is detected. By sensing the capacitance which exists between the electric fence and ground, instead of the resistance sensed according to the prior art, a higher sensitivity is obtained. Consequently, the invention relies on the understanding that the capacitance between the electric fence and ground is changed if an animal touches the electric fence and that such a change is detectable by means of different electric circuit solutions.
According to an embodiment of the present invention, a control unit is arranged to initiate said sensing discontinuously during relatively short recurrent time periods. In such a manner the energy consumption may be reduced significantly by the fact that the proximity detector does not operate continusouly.
According to a further embodiment of the present invention, the control unit is arranged to initiate application of said relatively high electric voltage in the form of at least one high voltage pulse.
The high voltage may thus be realised as one single high voltage pulse with a relatively high energy or as a series of high voltage pulses. By a pulse train of for instance 100 voltage pulses during 0.1 seconds, each voltage pulse may contain significantly less energy than if only one voltage pulse is utilised. The advantage of such a pulse train is that it may start only a few ms after an animal has been detected whereas a voltage pulse with a high energy requires a relatively long time period from the moment an animal is detected until the voltage pulse is released. In addition, the subjective feeling of uncomfort for the animal touching the electric fence appears to be greater by a pulse train than by a single voltage pulse.
According to a further embodiment of the invention, the proximity detector includes an interrupting member which is arranged to interrupt the connection of at least a part of the proximity detector to said electric fence when said relatively high electric voltage is applied to said fence. In such a manner it is possible to protect the electronic components which do not stand high voltages and which may be comprised by the proximity detector.
According to a further embodiment of the invention, the proximity detector is arranged to apply a signal to said fence, wherein said parameter includes the value of a the capacitance between said fence and ground. Since the capacitance is clearly changed already at an easy contact of the electric fence, it is advantageous to detect, by any suitable method, such capacitance changes as an indication that an animal touches the fence. Already the fact that an animal is close to the electric fence, for instance 1 mm, without touching the same, however, leads to a capacitance change which is detectable. Thereby, said signal may have a determined frequency and the proximity detector may form an electric circuit in which the capacitance of said fence is included in such a manner that a change of the capacitance causes a detectable phase displacement of the signal. Such a circuit may be realised by a resistance in series with the capacitance provided by said fence. Advantageously, the resistance is formed by the inner resistance of at least one MOS-switch which may be comprised by the the interrupting member. Advantageously, the proximity detector may include a signal generator for generating said signal and a phase detector, which is arranged to sense the phase deviation between the signal generated by the signal generator and the signal applied to said fence. In order to avoid the generation of a high voltage when no animal is in the proximity of the electric fence, the phase detector may be connected to a filter member which is arranged to filter very fast undesired phase deviations. Such phase deviations may for instance be caused by disturbances from power conduits. Furthermore, a high voltage protection may be provided between the phase detector and said fence in order to protect the electronic components included by the phase detector.
According to a further embodiment of the invention, the proximity detector includes an oscillator circuit in which the capacitance between said fence and ground determines the frequency of the circuit, wherein saied parameter includes the frequency of the osciallator circuit. By such a device the oscillation frequency of the oscillator circuit will be changed when an animal touches the electric fence. Such a frequency change may in an easy manner be detected by any suitable electronics, wherein the high voltage may be applied to the electric fence.
According to a further embodiment of the invention, the electric unit is arranged to be connectable to said fence by means of one single electric connection. In such a manner the inventive device is applicable to an existing metal fence in a very easy way. Moreover, it is not necessary that the electric fence forms a closed loop but it may be opened at both ends thereof. However, the electric fence may also form a closed loop, which is usual at different types of enclosures. By the invention the advantage is thereby obtained that if this circuit is interrupted all the functions of the electric fence will still operate.